1. Field of the Invention
This invention relates to a composite material for mounting fragile articles and, more particularly, a composite lead frame material on which semiconductor chips may be mounted with minimum thermal stress.
2. Description of the Prior Art
Medium and higher power semiconductor devices, such as transistors, must dissipate substantial amounts of energy. The package for each transistor must have low thermal resistance and not stress the semiconductor chip therein.
Copper meets the low thermal resistance, i.e., high thermal conductivity requirement, and so would be a good material on which to mount the chip. Unfortunately, its coefficient of thermal expansion is much greater than that of the semiconductor material, e.g., silicon, of the chip. Therefore, a chip mounted thereon will break or malfunction due to thermally induced stresses.
Iron-nickel-cobalt alloys have a thermal expansion coefficient close to silicon and induce much less stress. But these alloys have about 1/25th the conductivity of copper and cannot conduct the heat away. With no ability to carry the heat away, a semiconductor chip would soon overheat.
One solution has been to combine the two materials by inlaying iron-nickel-cobalt alloy in the copper and mounting the semiconductor chip on the iron-nickel-cobalt.
Another solution, as in the case of a gallium-arsenide laser diode, is to mount the diode on silver inlaid in a molybdenum base sufficiently heavy and strong to constrain the expansion of the silver.
But these bimetallic materials inherently bow or warp like any bimetallic materials in which the two metals have different coefficients of expansion. This bowing or warping will bend any article, such as a semiconductor chip, bonded to the bimetallic material.
Semiconductor chips are extremely brittle and bend very little before breaking. They become highly stressed causing characterization shifts and/or cracking and, therefore, will malfunction. As a result bending stresses must be minimized or avoided.
Some manufacturing processes require heat for bonding the semiconductor chip in place and heat is generated by operation of the device. For either or both reasons the bimetal strip warps and may stress or break the semiconductor chip.